The present invention generally relates to an atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved safety by reducing the potential risk of induced ventricular fibrillation which may otherwise result from the delivery of cardioverting electrical energy to the atria of the heart at the wrong time or under improper conditions. More specifically, the atrial defibrillator of the present invention provides greater assurance against applying cardioverting electrical energy to the atria of the heart under conditions believed to contribute to induced ventricular fibrillation by having both specific and sensitive R wave detection.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart, and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly, and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying cardioverting or defibrillating electrical energy to the heart in synchronism with a detected depolarization activation wave (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide relief to patients suffering from occurrences of atrial fibrillation. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages, which probably have been the cause of these defibrillators failing to become a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation, with one defibrillator requiring a visit to a physician to activate the defibrillator, and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved safety are fully described in U.S. Pat. No. 5,282,837, issued Feb. 1, 1994, in the names of John M. Adams and Clifton A. Alferness, for "Improved Atrial Defibrillator and Method", and U.S. Pat. No. 5,350,404, issued Sep. 27, 1994, in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen, for "Lead System for Use with an Atrial Defibrillator and Method", which patents are assigned to the assignee of the present invention and incorporated herein by reference. As disclosed in the aforementioned referenced patents, synchronizing the delivery of the defibrillating or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart is important to prevent induced ventricular fibrillation. ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. The timing of the delivery of the cardioverting energy to a detected R wave is very helpful in avoiding a T wave of the heart.
It has further been observed that during episodes of atrial fibrillation, the cardiac rate increases to a high rate and/or becomes extremely variable. At high cardiac rates, the R wave of each cardiac cycle becomes closely spaced to the T wave of the immediately preceding cardiac cycle. This may lead to a condition known in the art as an "R on T" condition. It is now believed that such a condition can contribute to induced ventricular fibrillation even if the atria are cardioverted in timed relation to a detected R wave.
U.S. Pat. No. 5,207,219, issued May 4, 1993, to John M. Adams, Clifton A. Alferness, Kenneth R. Infinger, and Joseph M. Bocek, which patent is assigned to the assignee of the present invention and incorporated herein by reference, discloses and claims an atrial defibrillator which solves this problem. As described in the above-referenced patent, this is accomplished by interval timing prior to applying the cardioverting or defibrillating electrical energy. The time interval between immediately successive R waves is timed by an interval timer and the cardioverting or defibrillating electrical energy is applied only when the interval timer times an interval which is greater than a minimum interval. This provides protection from the increased vulnerability to ventricular fibrillation resulting from a high cardiac rate.
To support the operation of an atrial defibrillator having both R wave synchronized cardioversion and interval timing, it would appear, at least intuitively, that extremely sensitive R wave detection would be required. In doing so, the reset of the interval timer with each R wave and the cardioversion in timed relation to an R wave would be assured.
Sensitive detection of R waves is consistent with, and even preferable in association with, interval timing. However, detection of R waves with high sensitivity, in reality, is not consistent with or preferred for synchronized cardioversion. Rather, detection of R waves with high specificity is preferred. As used herein, the term "sensitivity" is meant to denote the degree of ability to detect an actual event, such as an R wave (ventricular activation) of the heart, and the term "specificity" is meant to denote the degree of ability to reject non-actual events, such as non-R waves.
In view of the foregoing, the present invention provides an atrial defibrillator having R wave detection for supporting both synchronized cardioversion and interval timing. More specifically, a sensitive R wave detector assures that every R wave is detected for resetting the interval timing. Because this R wave detector is sensitive, it may also detect, and mistake for an R wave, other electrogram features, such as large T waves or premature ventricular contractions. However, because all actual R waves will be detected, other features which may be detected would only lend to further safety of the device by also resetting the interval timer.
In addition to the sensitive R wave detector, a specific R wave detector is provided to assure that cardioversion will be performed in timed relation to only actual R waves. This specific R wave detector may be made specific to such an extent that, in addition to all non-actual R waves being rejected, some actual R waves may also go undetected for synchronization. However, because it is important that cardioversion occur in timed relation to only an actual R wave, the occasional missing of an actual R wave will only delay the cardioversion for another cycle, or perhaps a few more cycles. This short delay in cardioversion is certainly tolerable in view of the advantages obtained by such a specific R wave detector.